The invention is to be used in connection with a metalization system for contacting silicon monolithic microelectronic devices commonly known as integrated circuits (IC's) and the fabrication of contracts thereon. The typical silicon IC substrate has devices fabricated therein and is ordinarily provided with an aluminum interconnection layer. This is then covered by a deposited insulating layer of silicon dioxide or phosphosilicate glass (PSG). This layer protects the aluminum interconnect layer as well as the microelectronic substrate against scratches, abrasion, and contaminants. It also can act as the insulator for a second conductive pattern deposited on top of the glass.
In beam lead and bump contact devices the glass layer is etched to produce holes through which the underlayer of aluminum can be contacted. A series of metal layers is then deposited to build up a substantial thickness. The outermost layer being usually gold or copper which can be bonded by means of heat and/or pressure to mating contacts on a circuit board or lead frame. Thus the metal deposited onto a microcircuit device is used not only to make contact thereto, but also to provide a suitable bonding medium for joining the device to other circuit elements.
One widely used metalization system consists of successive layers of aluminum, chromium, gold, nickel, and copper. The first aluminum layer contacts the aluminum layer under the glass coating through conventional etched holes. The chromium layer adheres well to the aluminum and acts as a barrier between the aluminum and successive metal layers. The chromium is covered with a thin layer of contact metal such as gold, a heavy layer of nickel, which acts as a hardening or stiffening layer, and an outer heavy layer of copper. The layers of metal are selectively deposited and/or etched to leave bumps of the composite layers over the contact regions of the microcircuit. The copending application of James M. Harris Ser. No. 578,653 was filed May 19, 1975, now U.S. Pat. No. 4,042,954, is titled METHOD FOR FORMING GANG BONDING BUMPS ON INTEGRATED CIRCUIT SEMICONDUCTOR DEVICES and discloses a particular metal system.
The copper bumps have proven to be an excellent medium for thermocompression bonding of the microelectronic chips to either package leads or printed wiring substrates. A major difficulty that has been encountered in the preferred metalization system is in the chromium etching. Chromium is difficult to etch and practically all chromium etches attack aluminum and copper excessively. One solution to this problem is to make the aluminum and copper layers sufficiently thick or massive so that after chromium etching there is still sufficient metal left to produce satisfactory results. However, this approach often results in an unsatisfactory bump shape and the aluminum attack can undercut the bump thereby making the contact mechanically unsound. Other solutions to the problem have produced other unsatisfactory side effects. Most of the approaches to the problem require careful control of the etching operation and this does not lend itself to batch processing. Desirably a number of semiconductor wafers will be processed simultaneously. This means that etching should be continued until all wafers are etched sufficiently. Where the etch is critical, some wafers will be overetched. Accordingly, in the prior art processes, a single wafer is critically etched with some sort of monitoring during etching to avoid overetching. This is costly both in terms of having to handle each wafer and in terms of having to monitor.